The field of this invention is the control of vehicle door latches. The invention particularly relates to an electronic control which provides a minimum of actuation noise that could be annoying to a vehicle operator or passenger.
Vehicle door latch systems of the prior art typically comprise a rotatable fork bolt which, as a door is manually closed, is engaged in a door open position by a striker and rotated by the striker to a door closed position in which the striker is captured. A detent holds the fork bolt in the closed position against the door opening force of the compressed door weather seals. To open the door, the detent is disengaged from the fork bolt; and the seal force pushes the door open, with the striker rotating the fork bolt into the door open position as it escapes. A handle is connected by a mechanical linkage to the detent so that an operator can initiate door unlatching; and a lock mechanism physically prevents activation when locked.
Electronic unlatch systems of the prior art use the same basic unlatch mechanism but provide an electric actuator, with unlatching initiated by activation of a switch rather than by a mechanical linkage. For example, U.S. Pat. No. 4,858,971 to Haag et al shows an electronic vehicle door lock/unlatch control in which an electric motor provides activation of a vehicle door unlatch mechanism when it is connected in direct circuit with a DC power source by closure of an operator activated switch. Locking/unlocking is provided by a FET connected in series with the motor and operator activated switch and a flip-flop which can be activated to an unlock state to permit current flow through the FET and a lock state to prevent such current flow. The operator activated switch is mounted on a door handle so that the operator can activate the switch and pull the door fully open when it is unlatched. However, as shown, the motor will be activated as long as the switch is closed; and activated motors are subjected to stress and make noise which can be annoying to the operator. Some systems of more recent design provide a shorter actuator activation period, regardless of how long the activation switch is pressed, by initiating activation in response to switch activation but controlling activation duration by an timing circuit.
However, vehicles operated in the winter in cold climates are occasionally subject to doors being frozen shut--a situation in which moisture on the outside of the door weather seals freezes and holds the door in the closed position to prevent rotation of the fork bolt when the latter is released by the detent. Such a door must be physically pulled open from its primary latch position; and the detent must be held in a fork bolt releasing position for up to several seconds to permit this to happen. This is easily done in the Haag et al system described above by the operator continuing to activate the switch, but the motor activation noise and actuator stress will continue throughout the activation. This is much less easily done in a system using a timed activation period, since such periods are typically very short. On the other hand, lengthening the activation period to provide easier frozen door opening results in greater noise and actuator stress in the vast majority of cases where the door is not frozen shut.